Root pressure is maximum at the time of

Root pressure is the osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves. It is primarily generated by the active transport of mineral nutrients into the root xylem, which lowers the water potential and causes water to diffuse from the soil into the root xylem due to osmosis. This accumulation of water in the xylem pushes on the rigid cells, creating root pressure.

Root pressure is typically highest during periods of low transpiration and high water absorption. Transpiration is the process by which water evaporates from the leaves, creating a negative pressure that pulls water up from the roots. When transpiration rates are low, such as at night or during periods of high humidity, the plant loses less water through evaporation. Simultaneously, if the plant continues to absorb water actively, the accumulation of water in the root xylem increases, leading to higher root pressure.

Therefore, the correct answer is:

d) high water absorption and low transpiration

This combination allows for the maximum buildup of root pressure, facilitating the upward movement of water and nutrients through the plant.